On Mon, Dec 2, 2024 at 10:42 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> commit e9e58a4ec3b1 ("memcg: avoid use cmpxchg in swap cgroup maintainance")
> replaced the cmpxchg/xchg with a global irq spinlock because some archs
> doesn't support 2 bytes cmpxchg/xchg. Clearly this won't scale well.
>
> And as commented in swap_cgroup.c, this lock is not needed for map
> synchronization.
>
> Emulation of 2 bytes cmpxchg/xchg with atomic isn't hard, so implement
> it to get rid of this lock.
>
> Testing using 64G brd and build with build kernel with make -j96 in 1.5G
> memory cgroup using 4k folios showed below improvement (10 test run):
>
> Before this series:
> Sys time: 10730.08 (stdev 49.030728)
> Real time: 171.03 (stdev 0.850355)
>
> After this commit:
> Sys time: 9612.24 (stdev 66.310789), -10.42%
> Real time: 159.78 (stdev 0.577193), -6.57%
>
> With 64k folios and 2G memcg:
> Before this series:
> Sys time: 7626.77 (stdev 43.545517)
> Real time: 136.22 (stdev 1.265544)
>
> After this commit:
> Sys time: 6936.03 (stdev 39.996280), -9.06%
> Real time: 129.65 (stdev 0.880039), -4.82%
>
> Sequential swapout of 8G 4k zero folios (24 test run):
> Before this series:
> 5461409.12 us (stdev 183957.827084)
>
> After this commit:
> 5420447.26 us (stdev 196419.240317)
>
> Sequential swapin of 8G 4k zero folios (24 test run):
> Before this series:
> 19736958.916667 us (stdev 189027.246676)
>
> After this commit:
> 19662182.629630 us (stdev 172717.640614)
>
> Performance is better or at least not worse for all tests above.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
> mm/swap_cgroup.c | 56 +++++++++++++++++++++++++++++++++++-------------
> 1 file changed, 41 insertions(+), 15 deletions(-)
>
> diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
> index a76afdc3666a..028f5e6be3f0 100644
> --- a/mm/swap_cgroup.c
> +++ b/mm/swap_cgroup.c
> @@ -5,6 +5,15 @@
>
> #include <linux/swapops.h> /* depends on mm.h include */
>
> +#define ID_PER_UNIT (sizeof(atomic_t) / sizeof(unsigned short))
You might want to have some compile time assert that (sizeof(atomic_t)
% sizeof(unsigned short)) is zero. Could not hurt.
> +struct swap_cgroup_unit {
> + union {
> + int raw;
> + atomic_t val;
> + unsigned short __id[ID_PER_UNIT];
> + };
> +};
I suggest just getting rid of this complicated struct/union and using
bit shift and mask to get the u16 out from the atomic_t.
> +
> static DEFINE_MUTEX(swap_cgroup_mutex);
>
> struct swap_cgroup {
> @@ -12,8 +21,10 @@ struct swap_cgroup {
> };
>
> struct swap_cgroup_ctrl {
> - unsigned short *map;
> - spinlock_t lock;
> + union {
> + struct swap_cgroup_unit *units;
> + unsigned short *map;
You really shouldn't access the map as an "unsigned short" array,
therefore, I suggest changing the array pointer to "atomic_t".
> + };
> };
>
> static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
> @@ -31,6 +42,24 @@ static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
> *
> * TODO: we can push these buffers out to HIGHMEM.
> */
> +static unsigned short __swap_cgroup_xchg(void *map,
> + pgoff_t offset,
> + unsigned int new_id)
> +{
> + unsigned int old_id;
> + struct swap_cgroup_unit *units = map;
> + struct swap_cgroup_unit *unit = &units[offset / ID_PER_UNIT];
> + struct swap_cgroup_unit new, old = { .raw = atomic_read(&unit->val) };
> +
> + do {
> + new.raw = old.raw;
> + old_id = old.__id[offset % ID_PER_UNIT];
> + new.__id[offset % ID_PER_UNIT] = new_id;
> + } while (!atomic_try_cmpxchg(&unit->val, &old.raw, new.raw));
I suggest just calculating the atomic_t offset (offset /
ID_PER_UNIT) and getting the address of the atomic_t.
Then use the mask and shift to construct the new atomic_t value. It is
likely to generate better code.
You don't want the compiler to generate memory load and store for
constructing the temporary new value.
I haven't checked the machine generated code, I suspect the compiler
is not smart enough to convert those into register shift here. Which
is what you really want.
> +
> + return old_id;
> +}
> +
> /**
> * swap_cgroup_record - record mem_cgroup for a set of swap entries
> * @ent: the first swap entry to be recorded into
> @@ -44,22 +73,19 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id,
> unsigned int nr_ents)
> {
> struct swap_cgroup_ctrl *ctrl;
> - unsigned short *map;
> - unsigned short old;
> - unsigned long flags;
> pgoff_t offset = swp_offset(ent);
> pgoff_t end = offset + nr_ents;
> + unsigned short old, iter;
> + unsigned short *map;
Make it an atomic_t pointer here as well.
>
> ctrl = &swap_cgroup_ctrl[swp_type(ent)];
> map = ctrl->map;
>
> - spin_lock_irqsave(&ctrl->lock, flags);
> - old = map[offset];
> + old = READ_ONCE(map[offset]);
Ah, you shouldn't perform u16 reading directly. That will get into the
endian problem of how the u16 is arranged into atomic_t. You should do
atomic reading then shift the bits out so you don't have the endian
problem. It is a bad idea mixing atomic updates and reading the middle
of the atomic address location.
Chris
> do {
> - VM_BUG_ON(map[offset] != old);
> - map[offset] = id;
> + iter = __swap_cgroup_xchg(map, offset, id);
> + VM_BUG_ON(iter != old);
> } while (++offset != end);
> - spin_unlock_irqrestore(&ctrl->lock, flags);
>
> return old;
> }
> @@ -85,20 +111,20 @@ unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
>
> int swap_cgroup_swapon(int type, unsigned long max_pages)
> {
> - void *map;
> + struct swap_cgroup_unit *units;
> struct swap_cgroup_ctrl *ctrl;
>
> if (mem_cgroup_disabled())
> return 0;
>
> - map = vzalloc(max_pages * sizeof(unsigned short));
> - if (!map)
> + units = vzalloc(DIV_ROUND_UP(max_pages, ID_PER_UNIT) *
> + sizeof(struct swap_cgroup_unit));
> + if (!units)
> goto nomem;
>
> ctrl = &swap_cgroup_ctrl[type];
> mutex_lock(&swap_cgroup_mutex);
> - ctrl->map = map;
> - spin_lock_init(&ctrl->lock);
> + ctrl->units = units;
> mutex_unlock(&swap_cgroup_mutex);
>
> return 0;
> --
> 2.47.0
>
